1. Field of the Invention
The invention relates to an interconnection (contact) element suitable for effective connections between electronic components.
2. Description of Related Art
Interconnection or contact elements may be used to connect devices of an electronic component or one electronic component to another electronic component. For example, an interconnection element may be used to connect two circuits of an integrated circuit chip or to connect an application specific integrated circuit (ASIC). Interconnection elements may also be used to connect the integrated circuit chip to a chip package suitable for mounting on a printed circuit board (PCB) of a computer or other electronic device or to connect the integrated circuit chip directly to the PCB. Interconnection elements may further be used to connect the integrated circuit chip to a test device such as a probe card assembly or other substrate to test the chip.
Generally, interconnection or contact elements between electronic components can be classified into at least the two broad categories of xe2x80x9crelatively permanentxe2x80x9d and xe2x80x9creadily demountable.xe2x80x9d
An example of a xe2x80x9crelatively permanentxe2x80x9d contact element is a wire bond. Once two electronic components are connected to one another by a bonding of an interconnection element to each electronic component, a process of unbending must be used to separate the components. A wire bond interconnection element, such as between an integrated circuit chip or die and inner leads of a chip or package (or inner ends of lead frame fingers) typically utilizes a xe2x80x9crelatively permanentxe2x80x9d interconnection element.
One example of a xe2x80x9creadily demountablexe2x80x9d interconnection element is the interconnection element between rigid pins of one electronic component received by resilient socket elements of another electronic component, for example, a spring-loaded LGA socket or a zero-insertion force socket. A second type of a xe2x80x9creadily demountablexe2x80x9d interconnection element is an interconnection element that itself is resilient or spring-like or mounted in or on a spring or resilient medium. An example of such an interconnection element is a tungsten needle or a micro-spring contact of a probe card component such as described in commonly assigned U.S. Pat. No. 5,974,662, titled xe2x80x9cMethod of Planarizing Tips of Probe Elements of a Probe Card Assembly,xe2x80x9d issued Nov. 2, 1999 (FFI-P06). The interconnection element of a probe card component is typically intended to effect a temporary pressure connection between an electronic component to which the interconnection element is mounted and terminals of a second electronic component, such as a semiconductor device under test.
With regard to spring interconnection elements, generally, a minimum contact force is desired to effect reliable pressure contact to an electronic component (e.g., to terminals of an electronic component). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per terminal) may be desired to effect a reliable electrical pressure connection to a terminal of an electronic component.
A second factor of interest with regard to spring interconnection elements is the shape and metallurgy of the portion of the interconnection element making pressure connection to the terminal of the electronic component. With respect to the tungsten needle as a spring interconnection element, for example, the contact end is limited by the metallurgy of the element (i.e., tungsten) and, as the tungsten needle becomes smaller in diameter, it becomes commensurately more difficult to control or establish a desired shape at the contact end.
In certain instances, interconnection elements themselves are not resilient, but rather are supported by a resilient membrane. Membrane probes exemplify this situation, where a plurality of microbumps are disposed on a resilient membrane. Again, the technology required to manufacture such contact elements limits the design choices for the shape and metallurgy of the contact portion of the contact elements.
Commonly-assigned U.S. patent application Ser. No. 08/152,812, filed Nov. 16, 1993 (now U.S. Pat. No. 5,476,211, issued Dec. 19, 1995) (FFI-P01), discloses methods for making spring interconnection elements. In a preferred embodiment, these spring interconnection elements, which are particularly useful for micro-electronic applications, involve mounting an end of a flexible elongate element (e.g., wire xe2x80x9cstemxe2x80x9d or xe2x80x9cskeleton,xe2x80x9d sometimes xe2x80x9ccorexe2x80x9d) to a terminal on an electronic component, and coating the flexible element and adjacent surface of the terminal with a xe2x80x9cshellxe2x80x9d of one or more materials. One of skill in the art can select a combination of thickness, yield strength, and elastic modulus of the flexible element and shell materials to provide satisfactory force-to-deflection characteristics of the resulting spring interconnection elements. Exemplary materials for the core element include gold. Exemplary materials for the coating include nickel and its alloys. The resulting spring interconnection element is suitably used to effect pressure, or demountable, interconnections between two or more electronic components, including semiconductor devices.
As electronic components get increasingly smaller and the spacing between terminals on the electronic components get increasingly tighter (the pitch gets increasingly finer), it becomes increasingly more difficult to fabricate interconnections suitable for making electrical connection to terminals of an electronic component. Co-pending and commonly-assigned U.S. patent application Ser. No. 08/802,054, filed Feb. 18, 1997, titled xe2x80x9cMicroelectronic Contact Structure, and Method of Making Samexe2x80x9d (FFI-P34) (incorporated herein in full by reference), and corresponding PCT application published Nov. 27, 1997 as WO97/44676, disclose a method of making spring interconnection elements through lithographic techniques. In one embodiment, that application discloses forming a spring interconnection element (including a spring interconnection element that is a cantilever beam) on a sacrificial substrate and then transferring and mounting the interconnection element to a terminal on an electronic component. In that disclosure, the spring interconnection element is formed in the substrate itself through etching techniques. In co-pending, commonly-assigned U.S. patent application Ser. No. 08/852,152, titled xe2x80x9cMicroelectronic Spring Contact Elementsxe2x80x9d (FFI-P35), spring interconnection elements are formed on a substrate, including a substrate that is an electronic component, by depositing and patterning a plurality of masking layers to form an opening corresponding to a shape embodied for the spring interconnection element, depositing conductive material in the opening made by the patterned masking layers, and removing the masking layer to form the free-standing spring interconnection element.
Co-pending and commonly-assigned U.S. patent application Ser. No. 09/023,859, titled xe2x80x9cMicroelectronic Contact Structures and Methods of Making Samexe2x80x9d (FFI-P047) describes an interconnection element having a base end portion (post component), a body portion (beam component) and a contact end portion (tip component) and methods of separately forming each portion and joining the post portion together as desired on an electronic component.
U.S. Pat. No. 5,613,861 (and its counterpart divisional U.S. Pat. No. 5,848,685) issued to Smith et al. disclose photolithographically patterned spring interconnection elements formed on a substrate with a body having an inherent stress gradient formed of a resilient (e.g., elastic) material such as chrome-molybdenum alloy or a nickel-zirconium alloy. The stress gradient causes an end of the body to bend away from the substrate in the shape of an arc when the end is freed from the substrate.
In order to achieve the desired shape of the body, Smith et al. must limit the thickness of the interconnection element described in U.S. Pat. No. 5,613,861. A limit on the thickness of the interconnection element limits the spring constant, k, of the interconnection element (k increases as thickness increases) particularly in state-of-the-art interconnection element arrays where the dimensions (e.g., length and width) of individual interconnection arrays are reduced to accommodate a corresponding increase in contact pad or terminal density. A reduction of the spring constant generally reduces the amount of load or force, F, that may be applied to resilient interconnection elements for a given deflection, x (k=F/x). Thus, such interconnection elements generally sustain at best a moderate contact force, which may not be enough to effect reliable pressure contact to an electronic component.
What is needed is a resilient interconnection element and a method of improving the resiliency of an interconnection element, particularly interconnection elements that are suitable for present fine-pitch electrical connections and that is/are scalable for future technologies. Also needed are improved methods of making resilient interconnection elements, particularly methods that are repeatable, consistent, and inexpensive.
A method of fabricating and using an interconnection element is disclosed. In one embodiment, the interconnection element includes a first element material adapted to be coupled to a substrate and a second element material coupled to the first element material. At least one of the first element material and the second element material comprises a material having a transformable property such that upon transformation, a shape of the interconnection element is deformed. One example is a first and/or second element material having a property such that a first volume of the material is adapted to be transformed to a second, different volume.
One portion of the interconnection element of the invention may be coupled to a substrate such as an electronic component and in electrical communication with a signal at the substrate. The interconnection element may adopt a variety of configurations to allow elastic deformation when interconnecting two electronic components. One preferred embodiment is a configuration of a cantilever where a free end of the interconnection element is available for electrical connection, such as to probe contact pads or terminals of another electronic component or electrically couple two electronic components.
In one aspect of the invention, the first element material and the second element material of the interconnection element are arranged in a configuration such that the second element material overlies the first element material and the first volume is greater than the second volume. In such case, the transformation to a smaller second volume causes the interconnection element to deform along some portion of its length. In the example where the interconnection element adopts a cantilever configuration on a substrate, the volume transformation will cause, in one embodiment, the free end of the interconnection element to deform away (i.e., deflect) from the substrate with the free end arcing toward the coupled base, thus providing a projecting interconnection element. The distance that an interconnection may deflect from the substrate may be limited by, for example, a travel stop so that, in one instance, a consistent deflection distance may be established for a plurality of interconnection elements on a substrate (or above the substrate). Once the shape of the interconnection element is deformed, the second element material may be retained or removed, to yield a bi-material or mono-material interconnection element, respectively.
By providing an interconnection element of a material that undergoes a property transformation to deform the interconnection element, the fabrication of interconnection elements on a substrate may be simplified over prior art methods. The bi-material or mono-material interconnection element may also serve as a precursor or core to a final interconnection structure, the final structure formed by incorporating additional materials or layers, e.g., resilient layers, to form the interconnection element.
An electronic assembly is also disclosed. In one embodiment, the electronic assembly includes a substrate having a plurality of contact nodes accessible on the substrate and a plurality of free-standing resilient interconnection elements coupled to the substrate in such a manner that an attachment element or a base of an interconnection element electrically contacts a corresponding one of the contact nodes. In another embodiment, the contact nodes are signal lines on or in the substrate. The interconnection element comprises, in one aspect, a first element material adapted to be coupled to a substrate, and a second element material coupled to the first element material. One of the first element material and the second element material comprises a material having a transformable property such that upon transformation, a shape of the interconnection element is deformed. One example is a first and/or second element material having a property such that a first volume of the material is adapted to be transformed to a different second volume.
A method is further disclosed. The method includes creating an interconnection element coupled to a substrate and comprising a first element material and a second element material; releasing the interconnection element from the substrate at one end; and transforming a property of one of the first element material and the second element material to deform the interconnection element. One example of a property transformation is transforming the volume of one of the first element material and the second element material from a first volume to a second, different volume. In one embodiment, the release of the one end of the interconnection element may occur before or after the transformation step.
In one aspect, the interconnection element may adopt a variety of configurations such as a cantilever where a free end of the interconnection element is available for electrical connection, such as to probe or electrically couple an electronic assembly. The second element material may overly the first element material such that, in one embodiment, a volume transformation, for example, results in the second volume being less than the first volume. In the case of a cantilever interconnection element, the free end of the interconnection element may deform upon transformation to move the free end of the contact away from the substrate. Following the transformation, the second element material may be removed to create a mono-material interconnection element. This latter operation is particularly useful where the remaining material is in a state where it will retain, at least more or less, the modified shape.
The interconnection element formed by the method of the invention may stand alone or may serve as a precursor to a final interconnection structure. In one embodiment, the method includes, after the transformation step, patterning a masking material over the substrate to have an opening exposing the surface of the interconnection element and coating a third element material, such as a resilient material, on the exposed surface of the interconnection element to increase the spring constant of the interconnection element. One suitable masking material is an electrophoretic resist material that may be introduced onto the surface of the substrate. One advantage of using the electrophoretic resist material as the masking material is that it may be introduced uniformly over the surface without disrupting (e.g., damaging) the interconnection element or elements on the substrate.
Products including the new contact elements can be secured to a second electronic component by means of pressure, as in a holding fixture or a probing apparatus for testing wafers. Products also can be secured more permanently, as by soldering, much as in conventional mounting of ICs to system boards.
Additional features, embodiments, and benefits will be evident in view of the figures and detailed description presented herein.